This invention relates generally to anion exchange resins and more particularly to the production of anion exchange resins having basic ion exchange groups based on imidazoles and the production of anion exchange membranes containing in dispersed state these anion exchange resins.
More specifically, this invention relates to a process for producing strongly basic exchangers which, by not utilizing aliphatic tertiary amines in the formation of the anion exchange groups, have been improved particularly with respect to the production process, environmental sanitation, and heat resistance of the product.
Heretofore, a typically representative, strongly basic exchange resin has been one containing a quaternary ammonium salt as the ion exchange group. This quaternary ammonium salt is formed by causing an aliphatic tertiary amine (particularly trimethylamine) to react with a haloalkyl group (particularly a chloromethyl group) introduced onto a cross-linked polymer (e.g., an addition polymer, a polyaddition polymer, or a polycondensation polymer).
The methods of producing these strongly basic exchange resins known heretofore have been accompanied by a number of problems as described below.
First, the production processes in the known methods, in general are complicated. More specifically, a most typical strongly basic exchange resin is produced by chloromethylating a cross-linked polymer (e.g., a polystyrene cross-linked with divinylbenzene) and converting the resulting product into a quaternary ammonium salt. However, it is necessary to carry out chloromethylation in the presence of a suitable solvent and a suitable catalyst, and this requirement and the necessity of removing the solvent and the catalyst give rise to complications in the process. Furthermore, trimethylamine is generally used for this conversion into a quaternary ammonium salt, but its use is not desirable for reasons of work environment, because this amine is highly malodorous.
Another problem encountered in the practice of the method of this character, in which an aliphatic tertiary amine, particularly trimethylamine, is used in the formation of an anion exchange group is the poor heat resistance of the strongly basic exchange resin obtained as the product. In the present state of the art and related industries, in which the use of strongly basic exchange resins under high temperatures is frequently required, as the range of applications of these resins expands, inadequate heat resistance not only imposes various restrictions on the uses of these resins but also gives rise to problems in the secondary working processes thereof.
More specifically, as one example of use of an ion exchange resin, the case wherein such a resin is used in the form of a membrane may be considered. One method of producing an ion exchange membrane comprises dispersing a finely particulate ion exchange resin in a thermoplastic resin matrix in membrane form. In this method, however, the membrane is produced by heating and melting (e.g., at a temperature of 180.degree. to 230.degree. C) the thermoplastic resin containing the dispersed ion exchange resin; if the ion exchange resin has poor heat resistance, it cannot withstand such heating and melting.
A heterogeneous ion exchange membrane obtained by dispersing a finely particulate ion exchange resin in a thermoplastic resin membrane matrix is satisfactory with respect to production cost, strength of the membrane, and handling characteristic in comparison with a homogeneous ion exchange membrane (specifically, produced by chloromethylating a cross-linked polymer membrane and converting it into a quaternary ammonium salt with an amine, such as trimethylamine), which is the structure obtained by rendering the ion exchange resin, itself, into membrane form. Accordingly, it can be said that poor heat resistance of an ion exchange resin to be dispersed is a great disadvantage.
The inadequate heat resistance of a strongly basic exchange resin, having a quaternary ammonium salt based on an aliphatic tertiary amine, may be considered to give rise to decomposition of the quaternary ammonium salt, due to Hofmann degradation under a high temperature.